1. Technical Field
The present invention relates generally to H-bridge systems and methods for driving electric loads, and more particularly to high-efficiency H-bridge regulating circuits and methods for driving an electric load such as a thermoelectric cooler (TEC) by using a linear amplifier to drive one end of the load and a switching amplifier to drive the other end of the load.
2. Discussion
In virtually every modern fiber-optic network, there are lasers and TECs. The TECs are required since the laser outputs are highly sensitive to temperature fluctuations. Therefore, at every point in the network where there is a laser beam generating device, such as a laser diode, or where there is a component which has the possibility of undesirable thermal expansion that would adversely impact performance of the network, a thermoelectric cooler is typically employed. For example, in the typical fiber-optic cable system, there may be dozens of laser beams which are all focused together onto a single optical fiber. In addition, a single cable may have several optical fibers running in it. At the locations where these multiple beams come together and are focused into one or several optical fibers in a single cable, there typically is also one or more thermoelectric coolers. These TECs are used to stabilize the temperature of the laser diodes and of other temperature-sensitive components. In addition, the fiber-optic cables may run for a few kilometers, or dozens, hundreds or thousands of kilometers. Because of optical beam attenuation by the fiber, repeater optical amplifiers are used to reamplify the optical beam and send it on its way along the optical fiber cable. Each repeater amplifier includes at least one diode laser as an optical power source and typically utilizes at least one TEC.
In various optical fiber cable installations, there are typically multiple diode lasers which generate the laser beams. To ensure highly stable operation, the each of the laser diodes is rigorously temperature-controlled, typically by a closed-loop controller having a TEC which has a thermistor or other heat-sensing device mounted on it. Some systems rely on sensing the wavelength of the laser beam for sensing and stabilizing the laser diode temperature.
As is well-known, to operate the TECs for the purpose of stabilizing the temperature with any precision requires a power control circuit to regulate the direction and magnitude of the electric current applied across the TEC. The control portion of the overall circuit, not including the power amplifiers to which the TEC is connected, may be very small and draw very little power. But typical maximum power requirements for driving a TEC in applications such as stabilizing the temperature of diode lasers, depending on the size of the TEC, are 1 amp at 2 volts (which equals 2 watts), 2.5 amps at 2.5 volts (which equals 7.5 watts), or 3.5 amps at 3.5 volts (which equals 12.25 watts).
As is well-known, TECs are solid-state heat pumps. They are usually configured as relatively small flat box-like devices and operate when a DC current is passed through the Peltier elements in them. They typically are two-wire devices that have two flat ceramic plates (e.g., top and bottom sides) which serve as the heat sinks, with Peltier elements sandwiched therebetween. Typically, one plate or side is used as the temperature-stabilized side, on which a device, such as a laser diode, is mounted. That side is normally called the cold side, since heat from this side is usually being removed by the TEC. In some cases, like a cold start-up or where the set point is above the ambient temperature, heat may be pumped to this side by the TEC. A TEC transfers heat from one plate or side to the other, using the Peltier effect, with the direction of heat transfer being dependent upon the direction of the current. So, TECs can be used for heating or cooling and the magnitude of the current controls the amount of heat transfer.
Solid-state devices, like thermistors, mounted to one side of the TEC, provide a reliable indication of temperature and/or temperature variation on that side. The other plate or side to which heat is dumped (the heated plate) typically does not need to be controlledxe2x80x94often the heat is simply dumped into the environment. When using thermistors as a temperature feedback device, conventional power control circuits for TEC units can stabilize temperature on the cold side or plate of the TEC to about xc2x10.0001 degree C. relative stability to a set-point temperature. The relative stability of the temperature required is often xc2x10.1xc2x0 C. to xc2x11xc2x0 C. for the laser diodes used in fiber optic network applications. In some other cases, such as diode pumped lasers, the temperature stability required may be xc2x10.01xc2x0 C. or even xc2x10.001xc2x0 C. The size of TECs can be from 1 millimeter (mm) by 2 mm to 90 mm by 90 or more, with the height being from 2 mm to 6 mm or more. The size of the TECs often used in fiber-optic network applications typically varies from about 2 millimeters (mm) by 2 mm up to about 20 mm by 20 mm, with the height of the TECs typically being in the range of 2.5 mm to about 5 mm.
It is common today to use reasonably precise linear amplifiers to operate or drive TECs. Such linear mode TEC controllers are of low efficiency, often 20% to 40% efficient at most. The other 80% to 60% of the power applied to the linear amplifier drivers becomes waste heat which must be dumped to the ambient environment or otherwise removed. When many of these low efficiency TEC controllers are used in a single location, such as within a small enclosure, it is like having many multi-watt light bulbs burning inside that enclosure. This heat load in turn must be removed by appropriate types of heat removal systems, including fans, air-cooled or liquid-cooled heat sinks and/or air conditioning systems. These heat removal systems are an integral part of, and add significantly to the electric utility costs of operating, fiber-optic network equipment. Further, individual hot spots within such TEC-laden controller enclosures are another source of problems. The hot spots occur more often when the TECs are deployed in stacked configurations, since one TEC often transfers its heat to another TEC in the stack. Further, excess heat generally contributes to with long-term temperature stability problems. As is well known, the performance of the power supplies and/or control circuits can degrade due to excess heat over long periods of time, such as three to five or more years, which reduces fiber-optic network stability and/or performance.
Linear mode amplifiers have long been known to have the two major related drawbacks, namely low efficiency and the generation of unwanted heat, which, as noted, can induce long-term stability problems. But, linear mode amplifiers have long been recognized as having their benefits as well, including low cost due to a small number of required components, and very low noise production since they operate linearly.
Another benefit is that linear amplifiers take up less printed circuit board space, because they use a fewer number of components, and because the components used are typically smaller in size. For example, virtually all switch mode amplifiers have at least one big inductor and at least one big filter capacitor, while linear mode amplifiers normally do not. In the switching amplifiers, the filter capacitor is used to attenuate noise spikes generated by the switching actions of the output circuit, which generates large ripple currents in the output inductor. A well-known benefit of switch mode amplifiers is that they have high efficiency, which the linear amplifiers simply do not have since they typically are operated in their linear region.
In the telecommunications industry, network reliability is a very important characteristic which is much sought after. As more and more communications traffic and data flow across fiber-optic networks, the desire for long-term reliability has increased. Two of the main problems to be solved in this regard are: (a) the generation of heat through the operation of the laser diodes, the TECs and their associated lower efficiency controller/drivers; and (b) the larger power supply equipment needed to drive the TECs when using lower efficiency driving circuits and the larger heat removal equipment required to remove the excess heat that is produced by the driving circuits and this larger power supply equipment, and (c) the extra electrical energy consumed by the lower efficiency driving circuits, and the larger power supply equipment and larger heat removal equipment.
Therefore, it is a principal object of the present invention to provide a highly efficient H bridge regulating system for driving an electric load, such as a TEC, to thereby reduce electrical power required for driving the load. It is a related object to reduce the amount of heat and energy associated with operating and controlling a TEC or other electric load using an H bridge circuit. A further related object is to improve the reliability of the amplifiers and controllers used in H bridge drive systems by reducing the amount of heat generated during operation of the amplifiers and controllers.
It is another object to provide an H bridge regulating system for driving a two-wire (i.e., two-terminal) electric load, such as a TEC, by way of a linear amplifier and a switch mode amplifier connected, directly and simultaneously, to the opposite ends of the two-wire load. It is a related object to provide an H bridge regulating system that includes a linear amplifier stage and switching amplifier stage, and to provide a method for monitoring the outputs of both the linear amplifier and the switch mode amplifier simultaneously and controlling the outputs of both amplifiers in part based on one or more monitored conditions. A related object is to provide low-cost feedback circuits for implementing such monitoring and control functions.
Yet another object of the present invention is reduce the overall energy requirements needed to drive electric loads by providing a highly-efficient H-bridge circuit and method for doing so. A related object is reduce the amount of heating, and therefore the amount of air-conditioning and/or heat removal, that must be provided in electrical controller enclosures due to waste heat being generated across drive circuit components, particularly the power amplifier components and their associated power supplies.
In light of the foregoing objects, and in order to solve one or more of the foregoing problems, there is provided, according to a first aspect of the present invention, a novel H bridge regulating system for efficiently driving an electric load connected thereto through two terminal ends. The H bridge system comprises: a linear output stage and a switched output stage connected respectively to first and second opposite ends of an electric load. The linear output stage has a linear mode amplifier and a feedback circuit, preferably implemented with hard-wired components, which may respectively be called the first amplifier and the first feedback circuit. The feedback circuit preferably has at least a first conditioning element, which may be a resistor (R2). The linear mode amplifier generates an output signal (V1) at its first output, which is delivered directly to the first end of the electric load. The linear mode amplifier preferably has inverting and non-inverting inputs. A first feedback signal indicative of a first condition, such as a first output voltage (V1), relating to a condition (such as voltage) at the first end of the load preferably passes through the first conditioning element (R2) of the first feedback circuit to the inverting input of the linear mode amplifier.
In similar fashion, the switch mode output stage has a switch mode amplifier and feedback circuit, preferably implemented with hard-wired components, which may respectively be called the second amplifier and the second feedback circuit. The second feedback circuit preferably also has at least a first conditioning element, which may be a resistor (R4). The switch mode amplifier generates an output signal (V2) at its output, which may be called the second output, which is delivered to the second terminal end of the electric load. The switch mode amplifier preferably has inverting and non-inverting inputs. A second feedback signal indicative of a second condition, such as a second output voltage (V2), relating to a condition (such as voltage) at the second end of the load preferably passes through the first conditioning element (R4) of the second feedback circuit to the inverting input of the switch mode amplifier. A reference input signal, such as a command voltage (Vin), is preferably supplied to a non-inverting input of the linear mode amplifier, and is also supplied through a summing junction to the inverting input of the switch mode amplifier. The outputs from the linear mode amplifier and the switch mode amplifier are simultaneously applied to the electric load. Also, the voltage difference between those two outputs (V1 and V2), which translates to the voltage across the load (VLoad=V1xe2x88x92V2), is dictated by the reference input signal (Vin). This voltage across the load typically is some desired fraction of the total voltage between the positive and negative voltage supply rails (that is Vps and xe2x88x92Vps), as determined by the magnitude of the input signal, and may be negative or positive.
In the preferred embodiment, there are preferably further elements which form part of the first and second feedback circuits. In the first feedback circuit, there is preferably a second element, which may be a resistor (R1), for helping set the gain of the first amplifier. This second element (R1) is preferably tied to the ground, that is to a point midway between the power supply voltages Vps and xe2x88x92Vps provided to the first amplifier. In the second feedback circuit, there is preferably a second element, which may be a resistor (R6), connected to and helping establish a second input voltage applied to the non-inverting input of the second amplifier. The other side of this second element (R6) is also preferably tied to ground, that is to a point midway between-the power supply voltages Vps and xe2x88x92Vps provided to the second amplifier. In the second feedback circuit, there is also preferably a third conditioning element, which may also be a resistor (R3), that connects the input command signal (Vin) to the inverting input of the second or switching amplifier. In the second feedback circuit, there is also preferably a fourth conditioning element, which may also be a resistor (R5), that connects the first output (V1) to the non-inverting input of the switch mode amplifier.
As will be further explained below, the H bridge regulating system described above can, when proper ratios exist between the feedback circuit components, implement the classic H bridge transfer function across the load. Further, this system as described herein is of higher efficiency than conventional high-efficiency H bridges which use two switch mode output stages. Further, as will be explained below, as the gain of the linear amplifier is made large, the linear output stage will consume less power, resulting in a H bridge circuit that is higher in efficiency than one using two switch mode output stages.
This new H bridge system is of lower cost because one of the two switch mode output stages in the conventional design is replaced by a lower cost linear output stage. Also, the new system fits into a smaller package because of the use of the linear output stage, which takes less space to implement than the switch mode output stage it replaces.
The first output, that is the output of the linear mode amplifier, has three separate states, represented by left, central and right regions on the graphs. This will be further described below with respect to graphs of signal waveforms presented in FIGS. 3 and 4. The first and second output stages may operate between two power supply voltages, namely a positive supply voltage (Vps), which may be called the positive rail, and a negative supply voltage (xe2x88x92Vps), which may be called the negative rail. FIG. 3 shows the novel output stage waveforms and methods of the present invention in such an environment. In certain conventional H bridge circuit installations, it is common to omit one of the power supplies, for example, the negative power supply, to save cost, and simply tie the negative rail connections of the two identical output stages to ground. FIG. 4 shows the novel output stage waveforms and methods of the present invention in such an environment. Since the circuits and methods of the present invention readily work in both of these environments, it is convenient to give easy-to-remember names to the three separate states or regions of the linear amplifier""s output as shown on the graphs. The left region can be called the xe2x80x9clinear stage saturated to negative railxe2x80x9d region, the central region can be called the xe2x80x9clinear stage at linearxe2x80x9d region, and the right region can be called the xe2x80x9clinear stage saturated to positive railxe2x80x9d region. For convenience, these left, central and right regions may sometimes be respectively also be called the xe2x80x9clinear saturated to groundxe2x80x9d region (especially for the FIG. 4 environment), the xe2x80x9clinear-linearxe2x80x9d region, and the xe2x80x9clinear saturated to positivexe2x80x9d region.
As shown in the graph of FIG. 3B, which illustrates the two power supplies environment, the first output, that is the output of the linear mode amplifier, is held at xe2x88x92Vps in the left region by saturating the lower half of the first amplifier, that is sinking the first output to the negative rail. As shown in the graph of FIG. 4A, which illustrates the one power supply environment with the negative rail being to ground, the output of the linear mode amplifier is held at ground potential, that is zero volts, in the left region by saturating the lower half of the first amplifier, that is sinking the first output to its negative rail, which is tied to ground. In the right region, as shown in both FIG. 3C and FIG. 4A, the output of the linear mode amplifier is held at the positive supply voltage or positive rail voltage in the right regionxe2x80x94hence its name the xe2x80x9clinear stage saturated to positive railxe2x80x9d region. This, of course, is accomplished by saturating the upper half of the first amplifier, which of course effectively ties the first output to the positive rail. In both environments, the central region on the graphs of FIGS. 3B and 4A, is called the xe2x80x9clinear stage at linearxe2x80x9d region, because the linear mode amplifier operates linearly when in this region. Typically, both the upper and lower halves of the first or linear amplifier are operating in their active, conducting but non-saturated portions of their transistor power curves.
As will be further explained, by making the gain K of the linear amplifier sufficiently high, the central or xe2x80x9clinear-linearxe2x80x9d region may be made quite small, such as less than one percent of the overall range of operation between the ground and positive rail. Thus, the average power efficiency of the linear output stage can be very high most of the time, since it generally will consume very little power most of time as it is saturated to either one of the power supply rails. In this regard, it is strongly preferred that, in the H bridge circuits of the present invention, linear amplifiers which have a low saturation voltage or a low saturation resistance be utilized, since they will consume less power when saturated. By operating the linear output stage in a saturated state 99% or more of the time, the H bridge circuits of the present invention achieve overall efficiencies higher than conventional H bridge circuits having two identical switch-mode output stages, even when they are of even the highest efficiency.
As will be further explained, it is the small size of the central region on the graph, relative to the overall size or range of H bridge output voltages on the graph, that helps make the efficiency of this new H bridge design even more efficient. Preferably, this central region represents less than about ten percent, and more preferably less than about five percent, and most preferably about one percent or less, of the total active range of operation comprised of the three regions together. Further, the central location of this central region on the graph is selected to minimize the voltage across the load when the H bridge circuit is in this region. Note that the power consumed by the linear mode amplifier in the xe2x80x9clinear-linearxe2x80x9d region is the product of the current times the voltage drop across that half of the linear mode amplifier that is conducting). The voltage across the load is very small in this region, for example less than 100 millivolts. Thus, the amount of current flowing through the load (such as a TEC) will also be very small, which yields very small power consumption. The power consumption is the product of the load current and the voltage drop (a) between the linear output and the positive power rail if the load current is positive (that is, if the current flows from the V1 end to the V2 end), or (b) between the linear output and the negative rail (which may, as noted above, be tied to ground in the one power supply environment). Thus, the linear amplifier generates very little waste heat, even in its active state. Accordingly, the linear amplifier stage of the present invention, in comparison to the usual arrangement of a conventional H bridge made of two linear amplifiers, requires (1) a much smaller heat sink, or (2) perhaps even no heat sink. Either way, this helps reduce the cost of constructing a commercially viable H bridge circuit based upon the present invention.
The foregoing objects and advantages of the H-bridge regulating system and method of the present invention, together with the structural characteristics and operational details thereof, which have been only briefly summarized in the foregoing passages. They will become more apparent to those skilled in the art upon reading the detailed description of the preferred embodiments which follows in this specification, together with the illustrations presented in the accompanying figures.